There's a new "ride" at
Disneyland. Not too dramatic for visitors, but remarkable from a scientific standpoint.
The entire theme park moves up and down every year. Just a couple of centimeters. No extra
charge.

Nearby, the annual swelling of the ground is far worse.

A new satellite-based study shows parts of the Santa Ana basin, near Los Angeles, rise
and fall more than 4 inches (11 centimeters) every year. That would be fine if the land
masses returned to some equilibrium. Instead, over time, the elevation of much of the land
in the greater Los Angeles area and beyond is gradually dropping as a result of the land's
seasonal movements.

Vast areas of the Golden State are sinking, threatening to disrupt the flow of water
and sewage. It's a growing problem not confined to California, and difficult to spot in
other parts of the country.

Nature has little to do with it.

For Los Angeles, as ever, it's about water. Local water districts that purchase water
from outside the area pump it into huge, natural underground reservoirs to store it in
winter months. These aquifers, as they are called, are regions of rock, sand and soil that
can extend for many square miles and through which water flows freely.

Every summer, the water is withdrawn from these subterranean banks. But each year, the
withdrawals exceed the deposits.

"Every year when they pump water out, they're pumping it to a new, lower
level," said Gerald Bawden, a U.S. Geological Survey scientist who led the new study,
which appears in the Aug. 23 issue of the journal Nature.

The seasonal draining causes rock and soil to compact to an extent that prevents an
aquifer from ever filling to its previous capacity -- something scientists have understood
since the late 1920s. Like a hardened sponge, the compacted material simply can't hold
water as well as before.

The result is that in addition to the seasonal fluctuation, the ground in some places
also sinks steadily over time. It's a process geologists call subsidence.

Faulty fault data

Potentially the greatest impact of the process is that it renders meaningless some of
the important data used to study earthquake potential in the region. The Global
Positioning System (GPS), which geologists rely on to monitor long-term ground movement
along earthquake faults that might presage sudden and deadly quakes, is being fooled by
the thirst of millions of people.

After a pair of large Southern California earthquakes, including the Northridge temblor
that killed 56 peoplein 1994, geologists installed 250 GPS stations along faults
throughout the region to monitor ground movement.

Plates of the Earth's crust are known to creep over the years, and the movement can be
detected along fault lines, where two plates meet and move in different directions. This
creep is known to produce stress in the rocks near the fault. When the stress builds to a
certain point, it breaks, and an earthquake results.

But the seasonal swelling caused by the pumping of water, as well as withdrawals of oil
and natural gas in the area, obscure and in some cases mimic the tectonic signals detected
by the stations, the new study found.

As the ground rises and falls over an aquifer, it can actually pull at the edges,
creating horizontal movement near the perimeter of the aquifer. This relationship is
important because earthquake faults often form the boundaries of an aquifer. Along a
fault, rock is ground into a fine powder the consistency of potter's clay, and water
cannot penetrate this soil. Fault lines can extend a mile or more below the surface.

So the horizontal ground movement caused by seasonal water pumping sometimes occurs
right where GPS data collection is most critical -- along the fault.

Working around the problem

Bawden said the new findings will make it more challenging for geologists to study
fault movement in Southern California.

"It's not unrecoverable," he said. "We can still learn what's going on.
It just has been made a lot more difficult."

One way to work around the problem is to ignore areas where groundwater pumping alters
the surface, and instead take measurements in places that are not affected by the problem.
And since fault lines crisscross much of California, pockets of ground can be identified
where there is no effect from aquifers. Another solution would be to figure out the
pattern of seasonal variation over the next three or four years and account for it.

It's the long-term sinking that will be the most challenging to separate from
fault-movement data, and Bawden working on ways to mathematically account for it.

But it will take several years, he said, to accumulate enough information and develop
confidence in whether ground movement is caused by human-induced long-term or seasonal
fluctuation or a creeping earthquake fault.

Sewers don't flow uphill

Devin Galloway, a USGS scientist who
specializes in subsidence but did not participate in the new study, said he was surprised
by the extent of both seasonal and long-term movement caused by groundwater siphoning.
Galloway said the study, which he has read, was based on solid methods and good data.

"Water managers should be aware of it," Galloway said. "But subsidence
is one of those issues that's really hard to get on peoples' radar screens. It's so
subtle, that people don't really notice it unless something happens."

They may notice before too long.

"The city of Santa Ana [just south of L.A. proper] is sinking at the rate of about
a half-inch a year," Bawden said. California's huge system of aqueducts, used to move
water into the region from Northern California and from the Colorado River, currently
slopes just 4 inches per mile to provide enough grade to keep water flowing.

"It's not going to take too many years before water is not flowing the proper
direction," he said.

Stephen Kashiwada, chief of the Division of Operations and Maintenance for the
California Department of Water Resources, said mild subsidence is not a problem. Pumping
stations help move water along, he said, and the canals are surveyed each year to check
for any change.

But sewer pipes typically rely on gravity alone, and they may soon be fighting an
uphill battle to get waste out of sunken areas. In the more distant future, a depression
could form that would cause portions of the Santa Ana river to become a lake, Bawden said.

If it comes to that, no one can say there were no warnings. As early as the mid-1950s,
geologists noted that groundwater pumping near the Santa Ana river had caused a primary
aquifer to drop below sea level, and saltwater had flowed into the aquifer up to 5 miles
inland.

To fight this problem, a series of 23 wells were drilled so that officials could pump
up to 1.3 million gallons of water into the ground each day. The effort creates a
hydraulic wall of sorts that keeps seawater at bay.

Elsewhere: That sinking feeling

Subsidence caused by groundwater pumping is nothing new. One region of California's
Central Valley, well north of the Los Angles area, is known to have subsided roughly 30
feet (9 meters) since the late 1940s. Other parts of the country are also affected.

In 1997, the USGS measured long-term sinking in select locations in or near these
cities:

West of Phoenix, Arizona: 18 feet (5.5 meters)

Houston, Texas: 9 feet (2.7 meters)

Las Vegas, Nevada: 6 feet (1.8 meters)

Measurable side effects have already occurred. At Edwards Air Force Base in California,
a giant crack in the ground caused by groundwater siphoning once rendered a backup runway
designed for the space shuttle unusable.

Bob Pierotti saw that huge fissure back in the early 1990s. He called it "the
mother of all fissures, a big gaping crack" that was some 20 feet wide and hundreds
of yards long. Pierotti is a geologist with the southern division of the California
Department of Water Resources. He says that in general, subsidence is viewed seriously by
those who manage water districts.

He said studies like the new one provide valuable information for water managers
charged with planning current and future water storage practices.

But addressing subsidence in the Santa Ana basin would require the cooperation of many
agencies. There are several water districts in the greater Los Angeles area, some
operating locally and some regionally, with multiple layers of bureaucracy.

Under your feet, but undetected

Subsidence is not confined to the West or to the areas studied so far. It could be
happening under your feet, too, geologists say, especially if you live above an aquifer
and near a highly populated area or in an agricultural region. Much of southern and
coastal New Jersey, for example, sits above a large underground water system.

Wells have pumped the New Jersey groundwater to the point that seawater backs into the
aquifer, rendering the water undrinkable in many low-lying places far inland.

Such thirst is a growing problem around the country.

Since the mid-1950's, the amount of water in the United States supplied by groundwater
pumping has grown from less than 20 percent of the total to more than 30 percent. Much of
this rise occurred in the Southwest, where the population burgeoned after World War II.
Officials expect the problem to grow as more rivers and lakes are tapped out.

But in many parts of the country, the effect is hard to spot. Bawden and his colleagues
used satellites to study ground movement over time by bouncing radar off the surface on
subsequent passes in the satellite's orbit. The technique does not work well in heavily
forested areas because the radar bounces off tree leaves instead of the ground.

For Californians, as well as lovers of Mickey Mouse, there is some comforting news.

For locals who worry that drinking tap water might actually help bring on the next Big
One, Bawden and his colleagues did a little investigating: They took a close at one active
fault and determined that the human activity "has no bearing" on the current
seismic activity of the fault.

Nor is Disneyland in any imminent danger. The annual rise and fall is over such a broad
area, Bawden said, that it doesn't threaten to cause any structural damage at the park.